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Biggest dark matter detector lies in wait for antisocial WIMPs

By Louisa Field

Don’t WIMP out DEAP

(Image: Snolab)

I FEEL a rush in my stomach as the cage drops and the pressure on my eardrums increases. I am travelling 2 kilometres underground in a metal box that will carry me to the front line in the search for dark matter. I am eager to see DEAP, the world’s most sensitive detector before it is sealed off forever – but right now I wish I could stay at sea level.

My guide, Jack Dunger from the University of Oxford, reassures me. “The cage is much scarier than the tunnel,” he says. “Once you are down there you will feel more normal.”

The stuff Dunger’s colleagues are so keen to find is anything but normal. Dark matter accounts for about 80 per cent of the universe’s matter, yet only makes itself felt through gravity. It keeps galaxies from flying apart, but has never appeared in a detector.

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DEAP, the Dark matter Experiment using Argon Pulse-shape discrimination, is based on the theory that dark matter is a weakly interacting massive particle. These antisocial WIMPs are difficult to detect because they are, as the name suggests, unwilling to play with other particles. However, physicists hope that the interactions are just rare, rather than non-existent.

To help shield the detector from cosmic rays, which can mimic WIMPs, DEAP is located in the Sudbury Neutrino Observatory (SNOLAB), 2 kilometres beneath the frozen Canadian wasteland in the deepest area of an active nickel mine. At the end of April, it will join other underground detectors worldwide in the race to find dark matter.

Dunger leads me through a maze of white sterile corridors that suddenly open up into a massive cavern. In front of me is DEAP itself – a huge ball, 3.4 metres across, hanging from the ceiling by its steel neck. Here, I meet Mark Kos and Peter Skensved of Queens University, Canada, and ask them why they think DEAP could succeed where its competitors have failed.

“The short flippant answer is that we built a better detector,” Skensved grins.

“DEAP has two things really going for it. First of all, it’s very large,” Kos adds. “When it starts running it will be the biggest dark matter detector in the world.”

When it starts running at the end of April, DEAP will be the biggest dark matter detector in the world

DEAP can hold more than 3 tonnes of liquid argon, although they expect only the innermost part of the chamber to record a collision between a WIMP and an argon nucleus. Such an interaction would make the nucleus recoil, sending out a tiny flash of light. Photodetectors surrounding the argon will pick up this flash and reveal the presence and properties of the WIMP that caused it.

“The more mass you have the higher the chance is that dark matter will interact in your detector, so this goes towards increasing the sensitivity,” Kos says. The second advantage is the detector’s depth and shielding, which protect it from background radioactivity in the surrounding materials, including the cavern rock. Because it is so far underground, SNOLAB provides 100 times as much shielding from the muons in cosmic rays as the lab at Gran Sasso in Italy. This is home to similar dark matter experiments such as XENON1T, which will start number-crunching in the autumn.

“Muons are problematic because they can break up nuclei, releasing neutrons, and neutrons look just like WIMPs when they interact in dark matter detectors,” Kos says.

The dearth of WIMP strikes in other experiments like XENON and the LUX detector in South Dakota has worried physicists. But those detectors are more sensitive to low-mass WIMPs, while DEAP will have world-leading sensitivity to the higher-mass WIMPs that some theories favour, Kos says.

“Most theoretical model predictions give a WIMP mass that is greater than 100 gigaelectronvolts, which happens to be a mass range where DEAP is more sensitive compared to other experiments,” he says.

At the end of my tour, I trudge back towards the cage with a flock of tired miners and physicists. An announcement over the loudspeakers brings everyone to a sudden halt&colon; “All stations, all stations.” Dunger looks at me apologetically. The cage is broken and we need to wait at the nearest refuge station until they can fix it.